1. Field of the Invention
Embodiments of the disclosure relate to an organic light emitting diode (OLED) display capable of improving display quality by accurately extracting a threshold voltage of a drive thin film transistor (TFT).
2. Discussion of the Related Art
Various flat panel displays whose weight and size are smaller than cathode ray tubes have been recently developed. Examples of the flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electroluminescence device.
Because the PDP has a simple structure and is manufactured through a simple process, the PDP has been considered as a display device providing a large-sized screen while having characteristics such as lightness in weight and a thin profile. However, the PDP has disadvantages such as low light emitting efficiency, low luminance, and high power consumption. A thin film transistor (TFT) LCD using a TFT as a switching element is the most widely used flat panel display. However, because the TFT LCD is not a self-emission display, the TFT LCD has a narrow viewing angle and a low response speed. The electroluminescence device is classified into an inorganic light emitting diode display and an organic light emitting diode (OLED) display depending on a material of an emitting layer. Because the OLED display is a self-emission display, the OLED display has characteristics such as a fast response speed, a high light emitting efficiency, a high luminance, and a wide viewing angle.
The OLED display, as shown in FIG. 1, includes an organic light emitting diode. The organic light emitting diode includes organic compound layers between an anode electrode and a cathode electrode. The organic compound layers include a hole injection layer HIL, a hole transport layer HTL, an emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL.
When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emitting layer EML and form an exciton. Hence, the emitting layer EML generates visible light.
In the OLED display, pixels each including the above-described organic light emitting diode are arranged in a matrix format, and a brightness of the pixels selected by scan pulses is controlled by a gray level of video data. In the OLED display, the pixels are selected by selectively turning on a TFT used as an active element and remain in a light emitting state due to a charging voltage of a storage capacitor.
FIG. 2 is an equivalent circuit diagram of a pixel in a related art OLED display.
As shown in FIG. 2, each of pixels of a related art active matrix type OLED display includes an organic light emitting diode OLED, a data line DL, a gate line GL crossing the data line DL, a switch TFT SW, a drive TFT DR, and a storage capacitor Cst. Each of the switch TFT SW and the drive TFT DR may be implemented as an N-type metal-oxide semiconductor field effect transistor (MOSFET).
As the switch TFT SW is turned on in response to a scan pulse received from the gate line GL, a current path between a source electrode and a drain electrode of the switch TFT SW is switched on. During on-time of the switch TFT SW, a data voltage received from the data line DL is applied to a gate electrode of the drive TFT DR and the storage capacitor Cst.
The drive TFT DR controls a current flowing in the organic light emitting diode OLED depending on a voltage difference between the gate electrode and a source electrode of the drive TFT DR.
The storage capacitor Cst stores the data voltage applied to an electrode at one side of the storage capacitor Cst and thus keeps the data voltage applied to the gate electrode of the drive TFT DR constant during 1 frame period.
The organic light emitting diode OLED has a structure shown in FIG. 1. The organic light emitting diode OLED is connected between the source electrode of the drive TFT DR and a high potential driving voltage source VDD.
A brightness of the pixel shown in FIG. 2 is proportional to the current flowing in the organic light emitting diode OLED as indicated in the following Equation 1. The current flowing in the organic light emitting diode OLED is determined by a voltage difference between a gate voltage and a source voltage of the drive TFT DR and a threshold voltage of the drive TFT DR.
                    Ioled        =                              k            2                    ⁢                                    (                              Vgs                -                Vth                            )                        2                                              [                  Equation          ⁢                                          ⁢          1                ]            
In the above Equation 1, loled indicates a driving current of the organic light emitting diode OLED, k a constant determined by a mobility and a parasitic capacitance of the drive TFT DR, Vgs a voltage difference between a gate voltage Vg and a source voltage Vs of the drive TFT DR, and Vth a threshold voltage of the drive TFT DR.
As indicated in the above Equation 1, the driving current loled of the organic light emitting diode OLED is greatly affected by the threshold voltage Vth of the drive TFT DR.
In the OLED display, non-uniformity of luminances of the pixels is generally caused by a difference between electrical properties of the drive TFTs including the threshold voltage. The difference between the electrical properties of the drive TFTs is caused by a backplane of a display panel. In a display panel using a low temperature polysilicon (LTPS) backplane, a difference between the electrical properties of the drive TFTs is caused by an excimer laser annealing (ELA) process. On the other hand, in a display panel using an amorphous silicon (a-Si) backplane, a difference between the electrical properties of the drive TFTs is caused by not a process but a difference between degradation levels of the drive TFTs. The difference between the degradation levels is caused because of a difference between gate-bias stresses of the gate electrodes of the drive TFTs, and the difference between gate-bias stresses causes the difference the threshold voltages of the drive TFTs.
When the same data is applied to the pixels, there is a difference between currents flowing in the organic light emitting diodes of the pixels because of the difference between the electrical properties of the drive TFTs. Accordingly, a method including extracting the threshold voltages of the drive TFTs, storing the extracted threshold voltages in a memory, and reflecting the stored threshold voltages in display data has been proposed. In the related art method, as shown in FIG. 3, a sample and hold block 1, an analog-to-digital converter (ADC) 2, and a memory 3 are used to extract the threshold voltages of the drive TFTs. Threshold voltages Vth1 to Vthk of the pixels on the same horizontal are simultaneously sampled in response to a sampling clock SC and then are sequentially extracted in response to holding clocks HC1 to HCk. The extracted threshold voltages Vth1 to Vthk are input to the ADC 2 via a common output node cno of the sample and hold block 1 and are converted into digital values D1˜Dk. Then, the digital values D1˜Dk are stored in the memory 3. The sample and hold block 1 includes a plurality of sampling switches simultaneously operating in response to the sampling clock SC and a plurality of holding switches individually operating in response to the holding clocks HC1 to HCk.
As shown in FIG. 4, at a time when logic levels of the holding clocks HC1 to HCk change, the logic levels of the holding clocks HC1 to HCk do not critically change as indicated by ‘a’ but gradually changes as indicated by ‘b’ because of an influence such as a parasitic capacitance existing in a switch and a line. Hence, in the related art method for extracting the threshold voltage, when the holding switches are switched on or off, the threshold voltages of the adjacent pixels are extracted in a state where the threshold voltages of the adjacent pixels partially overlap each other. Namely, an overlap period OVP of the threshold voltages is generated. Because the threshold voltages of the adjacent pixels are mixed in the overlap period OVP, it is almost impossible to accurately extract the threshold voltages.
Further, interference occurs between successively output threshold voltages at the common output node cno of the sample and hold block 1 because of the parasitic capacitance existing in the switch and the line. Because a charge component of a previously output threshold voltage remains in the switch or the line and acts as the parasitic capacitance, the previously output threshold voltage affects a currently output threshold voltage. Because the related art method for extracting the threshold voltage does not perform an operation capable of discharging the remaining charge components, it is almost impossible to accurately extract the threshold voltages.
Accordingly, there is a limit to an improvement in a display quality in the related art method for extracting the threshold voltage.